The present invention pertains to X-ray imaging apparatus, and in particular, to automatic brightness control systems for such apparatus.
During a fluoroscopic examination of a patient, an X-ray image is displayed on the screen of a video monitor. To produce this image, the X-rays passing through a patient are detected by an image intensifier tube, which converts the X-ray image into a visible light image. A video camera receives the visible light image from the intensifier tube and produces a video signal for the monitor, which displays the patient image.
When the X-ray beam scans different portions of the patient, the brightness of the video image will change due to variations in the attenuation of the X-ray beam as it passes through different thicknesses and densities of body tissue and bone. In order to compensate for these variations in image brightness, various automatic compensation systems have been devised. One such system is described in U.S. Pat. No. 4,703,496 entitled "Automatic X-Ray Imager Brightness Control" and issued to the same assignee as the present invention. When this X-ray apparatus was operated in the fluorography mode, the luminances of picture elements in each video image field were averaged to produce a signal having a voltage proportional to the average image brightness.
The average brightness measurement is used as a feedback signal to control the excitation of the X-ray tube and the video gain of the apparatus to maintain the video image brightness substantially constant at an optimum level. The brightness control circuit comprised three separate loops for regulating tube current, bias voltage and video gain. In the X-ray tube current control loop, the ratio of a reference voltage to the measured average brightness voltage was determined. If this brightness ratio did not equal unity, an X-ray tube current controller adjusted the current level to eliminate the deviation of the actual brightness from the reference level. A value proportionate to the adjusted current level was stored until another brightness ratio was calculated for the next video image field.
In the X-ray tube bias voltage control loop, an error ratio of the stored current level value to a defined current limit was derived. This error ratio was multiplied by the present image's brightness ratio to provide a bias voltage control ratio indicative of how much of the brightness error the bias voltage control loop is obliged to correct. The bias voltage control ratio was corrected for nonlinearity between bias voltage change, and image brightness change and the resulting corrected value formed a bias voltage command which adjusted the voltage applied to the X-ray tube anode.
The video gain control loop calculated a first ratio between the tube current command for the last video field and a maximum current command limit; and derived a second ratio of the brightness change resulting from the last bias voltage control command to a maximum brightness change factor. The result of multiplying the last two mentioned ratios with the present image's brightness ratio became a new video gain control signal. The new video gain control signal varied the f-stop of the video camera and the electronic gain which also affected the image brightness. As a result of the way in which the previous tube current and bias voltage levels were ratioed in the control system, the X-ray tube current, bias voltage and video gain were concurrently adjusted on a priority basis in the stated order.
The primary effect on brightness is obtained most desirably with tube current control, the secondary effect with tube bias voltage control. It is least desirable to adjust image brightness with video gain control, because in addition to brightening the displayed X-ray image, increasing the electronic gain also increased the intensity of noise artifacts affecting the image. As the noise increased, the display became "grainer" which was unsatisfactory to the user. This adverse effect often confused the operator who did not recognize deterioration in the display image as indicating that the X-ray system was approaching the limit of its imaging range at the selected dose level. The operator expected the image to become darker as the imaging limit approached, as occurred in systems without automatic brightness control.